Galvanic cell sign conventions

The negative sign of U in this equation is caused by the convention on the potential scale for galvanic cells while we consider in the absolute scale electron... 

The lUPAC convention is consistent with the signs that the electrodes actually develop in a galvanic cell. That is, in the Cu/Ag cell shown in Figure 18-4, the Cu electrode becomes electron rich (negative) owing to the tendency of Cu to be oxidized to Cu while the Ag electrode becomes electron deficient (positive) because of the tendency for Ag" to be reduced to Ag. As the galvanic cell discharges spontaneously, the silver electrode is the cathode, while the copper electrode is the anode. 

The signs follow from the work aspects of the cell galvanic cells are spontaneous and thus have AG < 0 and Er > 0, whereas electrolytic cells consume electrical work and thus have AG > 0 and Er < 0. Equation 4 establishes the thermodynamic convention for cell potentials used in Table 2. 

Figure 6.27. Sign convention for electrolytic and galvanic cell reaction. Note in particular the signs for W, AG, and AV.

An electrochemical cell diagram is conventionally arranged in such a way that electrons flow from left to right in the equation. Comparing this with eqn. (6.30), we obtain the following sign convention for a galvanic cell and an electrolytic cell, respectively. 

For a cell that can have both galvanic and electrolytic modes (such as a battery), the thermodynamic convention requires an interchange of voltmeter electrodes for practicality, whUe the practical convention requires a sign reversal in computational and theoretical treatments of the two modes. In practice, most electrochemists switch between the two conventimis automatically, but for computer calculations, the thermodynamic convention is preferred, with careful attention paid to converting laboratory data to the quantities needed for calculation. 

---